Molybdenum Trihydride Complexes: Computational Determinations of Hydrogen Positions and Rearrangement Mechanisms

Abstract

In crystal structures of the molybdenum complexes [(1,2,4-C5H2(t)Bu3)Mo(PMe3)2H3] (Cp(t)Bu3) and [(C5H(i)Pr4)Mo(PMe3)2H3] (Cp(i)Pr4), the Mo-bound hydrogen positions were resolved for Cp(t)Bu3, but not for Cp(i)Pr4. NMR experiments revealed the existence of an unknown mechanism for hydrogen atom exchange in these molecules, which can be "frozen out" for Cp(t)Bu3 but not for Cp(i)Pr4. Density functional theory calculations of the most stable conformations for both complexes in the gas phase and in a continuum solvent model indicate that the H's of the Cp(i)Pr4 complex are unresloved because of their disorder, which does not occur for Cp(t)Bu3. A corresponding examination of alternative rearrangement pathways shows that the rearrangements of the H's could occur by two mechanisms: parallel to the cyclopentadienyl (Cp) ring in a single step and perpendicular to the Cp ring in two steps. The parallel pathway is preferred for both molecules, but it has a lower energy barrier for Cp(i)Pr4 than for Cp(t)Bu3.

Molybdenum Trihydride Complexes: Computational Model of Oxidatively Induced Reductive Elimination of Dihydrogen

“…We have also tested the influence of changing the basis set from double-ζ 6-31+G(d,p)/SDD to triple-ζ 6-311+G(d,p)/ Lanl2TZ(f) in GP models for the B3LYP functional. Like our previous findings for the neutral complexes [Cp tBu MoH 3 ] and [Cp iPr MoH 3 ], 47 changing the basis set for these cation molybdenum complexes does not introduce any significant changes to the energy profiles but significantly increases the computation time; see section S.II.3 in the SI.…”

“…A very similar situation was observed earlier for neutral molybdenum trihydride complexes. 47 Nevertheless, the H1−H2 frequency lies between 700i and 600i cm −1 for TS1s and thus appears to be the correct transition state for the creation of dihydrogen. B3LYP and ωB97X-D produce similar energies for the first step of the reaction (TS1 and IM), while ωB97X-D produces higher energies than those from B3LYP for the remainder of the H 2 elimination.…”

“…These cations have hydrogen rearrangement mechanisms similar to those described by us for neutral molybdenum trihydride complexes. 47 Two hydrogen rearrangement pathways were identified: the one-step mechanism where hydrogen atoms move parallel to the Cp R ring and the two-step mechanism where hydrogen atoms move perpendicular to the Cp R ring. However, in the case of cations, the influence of solvation is opposite to what was described previously for the neutral complexes.…”

“…Recent work has shown a good performance of the ωB97X-D functional for transition-metal complexes, 45 while the B3LYP functional is still a very common functional with a good ratio of computing cost to accuracy. 46 Furthermore, our previous work 47 showed that the combination of a B3LYP or ωB97X-D functional with the basis set described above was an efficient and accurate choice for molybdenum trihydride complexes.…”

“…These cation species also show hydrogen rearrangement mechanisms (parallel and perpendicular to the Cp R ring) similar to those described previously for neutral molybdenum trihydrides. 47 Likewise, DFT models for the OIRE of H 2 from the cationic trihydride complexes have a slightly higher barrier for [Cp tBu MoH 3 ] + than for [Cp iPr MoH 3 ] + . The greatest influence on the energy profiles for the H 2 elimination step is a different steric packing of the substituents in the Cp R ring.…”